Virtual fencing may be used for protecting or securing a separation line against intrusion by unwanted persons or objects in applications where a physical fence is inadequate or impractical, such as over long distances or where the terrain is too rough, or the cost is too high. The virtual fence could be used to protect a border, or the perimeters of an enclosed security area such as an airport, a strategic site, a hospital or university campus, fields and farms, or even private houses and estates The virtual fence should provide warning about the intended intrusion, and should be able to provide information about the location and type of intrusion expected. Current solutions based on video camera imaging, and using signal processing to detect changes in those images, generally have a number of disadvantages which have limited their widespread deployment, especially for border use over long distances, or in regions where the terrain is rough. Such video systems may have high false alarm rates (FAR), limited capabilities for screening irrelevant intrusions such as by animals, significant power consumption, and they could be costly in capital expenses. A system which overcomes at least some of the disadvantages of such prior art systems and methods would therefore be advantageous.
In International Patent Application No. PCT/IL2009/000417 for “Intrusion Warning System”, incorporated herewith by reference in its entirety, there is described an intrusion detection system based on a method of detecting reflections from an array of individually distinguished light beams directed in predetermined direction into the field of view, using an array of detectors, each detector viewing a predetermined direction in the field of view. Any significant change in detected light is interpreted as a change in the features of the field of view being surveilled, which may be attributed to an intrusion. By identifying the specific light beam detected, and the detector in the array which detects the change in detected light, the spatial position of the intrusion can be determined as the crossing point of the identified light beam and the field of view of the detector detecting the change. Such systems essentially perform mapping of the field of view being surveilled, and can thus be used for terrain mapping and range-finding as well as for intrusion detection.
The method described in PCT/IL2009/000417 is a parallax method, using triangulation to determine the position of the intrusion. This is shown in FIG. 1, where the intrusion at point X is being detected by detector 10 detecting a change in the level of the light reflected from impingement of laser 30 on the point X in the field. The accuracy with which the intrusion position can be located is dependent on D, the distance to the intrusion, and d, the distance between the detector element and the laser diode emitting point, both of which are typically mounted on a vertical baseline post 12.
For a separation of 30 cm, and for a detector array having a pixel size such that the pixel field resolution is 15 mm, an intrusion at a distance of 200 meters can be detected with an accuracy of 10 m. Because of the square law relationship with distance, using the same system, an intrusion at 50 m can be detected with an accuracy of 0.62 m. In general, the greater the value of d/D, the greater is the accuracy of the location measurement. However, a large value of d means that the laser array and the detector array must be widely spaced, and the physical size of the instrument must also be large, and this may make the system cumbersome to install and use, and easy to detect by a potential intruder. There therefore exists a need for an intrusion detection system, or a terrain surveillance system providing similar performance to that described in PCT/IL2009/000417, but having a more compactly sized package.
The disclosures of each of the publications mentioned in this section and in other sections of the specification are hereby incorporated by reference, each in its entirety.